The present invention generally relates to optical semiconductor devices and more particularly to a high-speed photodetection device used in a high-speed and large-capacity optical fiber telecommunication system having a transmission rate of 10 Gbps or more, or 40 Gbps or more.
FIG. 1 shows a schematic cross-sectional diagram of a conventional avalanche photodiode 20 of back-illuminated type receiving an incident optical beam at a substrate and designed for flip-chip mounting. FIG. 2, on the other hand, is an enlarged cross-sectional diagram showing the circled region with an enlarged scale.
Referring to FIGS. 1 and 2, the avalanche photodiode 20 is constructed on an n-type InP substrate 1 and includes an n-type InP buffer layer 2 formed on the substrate 1, an InGaAs optical absorption layer 3 of low doping concentration formed on the buffer layer 2, and an n-type InP layer 4 is formed on the InGaAs optical absorption layer 3 with an intervening n-type InGaAsP graded layer 13 interposed therebetween, wherein the graded layer 13 fills the energy band discontinuity between the InGaAs layer 3 and the InP layer 4.
On the InP avalanche layer 4, there is provided an insulation film such as an SiN film not illustrated, and an ion implantation process of a p-type impurity element is conducted into the n-type InP layer 4 selectively through a ring-shaped window formed in such an insulation film. As a result, there is formed a guard ring 14 in the n-type InP layer 4.
Further, the SiN film is removed and another SiN film 5-1 shown in FIG. 2 is formed, and a p-type impurity element is introduced into the n-type InP layer 4 through a window formed in the SiN film 5-1. Thereby, there is formed a p-type InP region 6 inside the n-type InP layer 4 in a manner surrounded by the guard ring 14, and is also formed a multiplication region 4-1 under the p-type InP region in the n-type InP layer.
A detailed description of a p-side electrode will be made from hereon with reference to FIGS. 1 and 2.
Referring to FIGS. 1 and 2, there is formed a p-side ohmic electrode 7 of an Au/Zn alloy on the InP region 6, wherein the p-side electrode 7 has a peripheral part covered by another SiN film 5-2 provided so as to cover a ring-shaped region, in which the InP layer 4 is exposed between the SiN film 5-1 and the p-side electrode 7.
Further, the p-side ohmic electrode 7 is covered with a barrier metal layer 9 having a Ti/Pt laminated structure such that the barrier metal layer 9 makes a contact with the p-side ohmic electrode at the contact window formed in the SiN film 5-2 so as to expose the p-side ohmic electrode 7.
The barrier metal layer 9 carries thereon an Au pillar 10, and the Au pillar 10 carries thereon a solder bump 11. Thereby, the barrier metal layer 9 blocks the diffusion of Au between the p-side ohmic electrode 7 and the Au pillar 10 so as to prevent Au from diffusing to the InP region 6 through the contact window.
Further, as represented in FIG. 1, there is formed a groove exposing the n-type InP buffer layer 2 around the guard ring 14, and an n-type ohmic electrode 8 is provided in contact with the buffer layer 2 such that the n-type ohmic electrode 8 extends along the sidewall of the groove and reaches the surface of the n-type InP layer 4. Thereby, a barrier metal layer 9A similar to the barrier metal layer 9, an Au pillar 10A similar to the Au pillar 10 and a solder bump 11A similar to the solder bump 11 are formed on the n-side ohmic electrode 8 on the InP layer 4. Further, the exposed sidewall of the grove and the n-side ohmic electrode 8 are covered by an insulation film 5 formed of the SiN film 5-1 and the SiN film 5-2.
Further, a microlens 15 is formed on the bottom surface of the substrate 1 in the photodetector 20 of FIG. 1, wherein it can be seen that an antireflection coating 12 is provided further on the bottom surface of the substrate 1.
Next, the operation of the photodetector 20 of FIGS. 1-2 will be explained.
In operation, a reverse bias voltage is applied across the p-side ohmic electrode 7 and the n-side ohmic electrode 8, and a signal light having a wavelength near 1300 nm or a wavelength of 1450-1650 nm is injected into the bottom side of the substrate 1.
It should be noted that the InP crystal constituting the substrate 1 is transparent to the incident light of the foregoing wavelength, and the signal light thus injected reaches the optical absorption layer 3 without being absorbed. Thereby, the absorption of the signal light takes place exclusively in the optical absorption layer 3.
In such a photodetector, the frequency response is determined by the CR time constant given by a product of the device capacitance C and the load resistance R and further by the carrier transit time.
Thus, when an attempt is made to improve the frequency response of the photodetector in view of the fast transmission rate of 10 Gbps or 40 Gbps, it is necessary to reduce the carrier transit time in addition to the CR time constant.
Because the carrier transit time increases in proportion with the thickness of the optical absorption layer 3, it is necessary to reduce the thickness of the optical absorption layer 3 in order to achieve the improvement of frequency response by reducing the carrier transit time. On the other hand, such a decrease of thickness of the optical absorption layer 3 causes the problem of decreased quantum efficiency because of the incomplete absorption of the incident light by the optical absorption layer 3. In such a case, the responsitivity of photodetection is degraded.
In this way, frequency response and quantum efficiency (photoresponsitivity) are in the relationship of tradeoff, and it has been difficult to design a high-speed photodetector having an optimum optical absorption layer thickness.
In view of the situation noted above, the inventor of the present invention has conceived a high-speed photodetector 30 having a mirror on the n-type InP layer 4 of FIG. 1 or 2 so as to reflect back the signal light not absorbed by the optical absorption layer again to the optical absorption layer.
FIG. 3 shows the construction of the photodetector 30 noted above, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. It should be noted that FIG. 3 is an enlarged cross-sectional view corresponding to the part shown in FIG. 2.
Referring to FIG. 3, an SiN pattern 5-2A is formed on the p-type region 6 in the photodetector 30 as a result patterning of the SiN film 5-2, and a ring-shaped p-side electrode 7A of the Au/Zn alloy is formed in the ring-shaped opening between the SiN pattern 5-2A and the SiN film 5-2 in place of the p-side electrode 7.
On the p-side ring-shaped electrode 7A, there is formed a ring-shaped barrier metal pattern 9A of the Ti/Pt stacked structure so as to cover the ring-shaped p-side ohmic electrode 7A and so as to cover the peripheral part of the SiN pattern 5-2A as well as the peripheral part of the SiN pattern 5-2 along the ring-shaped opening. Further, the Au pillar 10 is provided on the barrier metal pattern 9A so as to make a contact with the SiN pattern 5-2A at the central opening. The Au pillar 10 thus formed carries thereon the solder bump 11 similarly to the construction of FIGS. 1 and 2.
In the photodetector 30 of FIG. 3, it should be noted that the signal light injected into the substrate 1 is reflected by a high-reflectivity mirror formed from the SiN pattern 5-2A and the Au pillar 10 and is returned to the optical absorption layer 13. Thus, it becomes possible to realize sufficient quantum efficiency in the device 30 of FIG. 3 even in the case the thickness of the optical absorption layer 3 is reduced.
In the photodetector 20 or 30 of FIGS. 1-3, the optical absorption layer 3 has an absorption edge wavelength xcexe of about 1.7 xcexcm and a sufficient absorption coefficient is secured for the wavelength band of 1550 nm called C band.
On the other hand, there arises a problem in that the absorption coefficient drops rapidly in the long wavelength band called L band, which is a wavelength band longer than 1580 nm. With this, there arises a problem that substantial degradation of responsitivity is inevitable in the L band even in the case the reflectivity of the mirror is improved or the wavelength-dependence of reflectivity is suppressed.
Accordingly, it is a general object of the present invention to provide a novel and useful photodetector wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a photodetector having a high frequency response and high responsitivity while simultaneously maintaining sufficient sensitivity for signal light of broad wavelength range used in a wavelength division multiplexing optical telecommunication system.
Another object of the present invention is to provide a high-speed optical detector having a sufficient responsitivity in an L band region, in which there occurs a rapid degradation of optical absorption coefficient in an optical absorption layer.
Another object of the present invention is to provide a photodetector having an optical absorption layer, said optical absorption layer having a bandgap Eg(eV), an absorption edge wavelength xcexe (xcexcM) and an optical absorption coefficient provided by:                                                                         α                ⁡                                  (                                      ϵ                    ⁡                                          (                      λ                      )                                                        )                                            =                            ⁢                              α                ⁡                                  (                  ξ                  )                                                                                                        =                            ⁢                                                                                          0.18510                                              -                        2                                                              ·                                          4.3410                      6                                                                            0.0124                                                  ·                                  k                                ·                                  [                                                                                    ξ                        (                                                  "LeftBracketingBar"                                                                                    A                              i                                                        ⁡                                                          (                                                              -                                ξ                                                            )                                                                                "RightBracketingBar"                                                )                                            2                                        +                                                                                                                                                                              ⁢                                      (                                          "LeftBracketingBar"                                                                        ⅆ                                                      ⅆ                            ξ                                                                          ⁢                                                                              A                            i                                                    ⁡                                                      (                                                          -                              ξ                                                        )                                                                                              "RightBracketingBar"                                        )                                    ]                                2                            ,                                                          (        A        )            
wherein                     A        i            ⁡              (                  -          ξ                )              =                  1                  π                    ⁢                        ∫          0          ∞                ⁢                              cos            ⁡                          [                                                                    1                    3                                    ⁢                                      t                    3                                                  +                                                      (                                          -                      ξ                                        )                                    ⁢                  t                                            ]                                ⁢                      ⅆ            t                                ,
Ai(xe2x88x92"xgr") being an Airy function,                                                         ϵ              ⁡                              (                λ                )                                      =                                                            E                  ⁡                                      (                    λ                    )                                                  -                Eg                            k                                ,                      
                    ⁢                                                                                                                E                      g                                        =                                          1.2398472447                                                                                                    n                            ⁡                                                          (                                                              λ                                ⁢                                                                  xe2x80x83                                                                ⁢                                e                                                            )                                                                                ·                          λ                                                ⁢                                                  xe2x80x83                                                ⁢                        e                                                                              ,                                                                                                  xe2x80x83                                    ⁢                                                                                    E                        ⁡                                                  (                          λ                          )                                                                    =                                              1.2398472447                                                                              n                            ⁡                                                          (                              λ                              )                                                                                ·                          λ                                                                                      ,                                                                                      ⁢                  
                ⁢                                            n              ⁡                              (                λ                )                                      =                          1              +                                                [                                      6432.8                    +                                          2949810                                              146                        -                                                  1                                                      λ                            2                                                                                                                +                                          25540                                              41                        -                                                  1                                                      λ                            2                                                                                                                                ]                                ·                                  10                                      -                    8                                                                                ,                                    (        B        )                                          k          =                                    0.0124              [                              V                d                            ]                                      2              3                                      ,                            (        C        )            
xcex5(xcex), n(xcex), E(xcex) being respectively a dielectric constant, a refractive index and a bandgap at the wavelength of an incident light, k being a coefficient obtained from a voltage V(V) applied to said optical absorption layer and a layer thickness d(m) of said optical absorption layer,
wherein said thickness d(m) of said optical absorption layer is set with respect to said voltage V(V) applied to said optical absorption layer so as to satisfy a relationship
106xe2x89xa6V/dxe2x89xa6107.xe2x80x83xe2x80x83(D)
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.
According to the present invention, it becomes possible to expand the operational wavelength band of a photodetector having an optical absorption layer of which optical absorption coefficient is given by Equations (A)-(C), in a longer wavelength side beyond a conventional limit of 1580 nm, by setting the thickness d of the optical absorption layer with respect to an applied voltage V in a range of 106xe2x89xa6V/dxe2x89xa6107. Thereby, it becomes possible to expand the operational wavelength band of a wavelength division multiplexing optical transmission system.
Other objects and further features of the present invention will become apparent from the following detailed description of the preferred embodiment when read in conjunction with the attached drawings.